# Odds ratios and 95% confidence intervals

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**This rbiostatistics Odds Ratio Plot does not calculate your odds ratio. You must complete this yourself. Once you have done this you can use this page to display your odds ratio as a plot. If you’d like a little more info about odds ratios and their use, please continue reading. **

What is an Odds Ratio?

Put simply, an odds ratio (OR) is the ratio of an outcome occurring following exposure compared to the odds of an outcome occurring in the absence of the same exposure.

You can calculate an odds ratio yourself using the below formula:

OR = (A/C) / (B/D)
A = Number of exposed cases
B = Number of exposed non-cases
C = Number of unexposed cases
D = Number of unexposed non-cases

When would I use an odds ratio?

Odds ratios are most commonly used to report the findings of case-control studies however they are also used within cross-sectional and cohort study designs, albeit with modifications.

Interpreting an odds ratio

An OR greater than 1.0 indicates an increase in odds of outcome in the exposed group, an OR less than 1.0 indicates a decrease in odds. If it is equal to 1.0 there is no difference between groups.

Odds ratios can also be presented in the form of a confidence interval.

What is a Confidence Interval?

A 95% Confidence Interval provides an estimate of the precision of the odds ratio. It describes a 95% probability that the true population value is within the odds ratio observed within your sample data.

• Odds ratios provide a simple measure of association between exposure and outcome
• Odds ratios don’t require any particularly sophisticated data or steps in analysis
• Odds ratios presented as plots are fairly intuitive and easy to interpret

• Odds ratios are often confused with relative risk. The difference between these two statistical concepts goes beyond the scope of this article but it is an important distinction to be aware of.
• If the outcome is not rare (it is common) using an odds ratio will overstate the effect of exposure on outcome.

Worked Example

1. Click ↬ Analyse
2. Upload your excel file with the same formatting as the example plot format
3. Select the Lower, Midpoint, Upper values and Table text
4. Select and view the plot

Written By Daniel Richardson

Resources

https://www.theanalysisfactor.com/the-difference-between-relative-risk-and-odds-ratios/

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